mu4e: Replace mu4e-view-use-gnus with mu4e-view-use-old

Make mu4e-view-use-gnus obsolete (it's the default now), and add a
variable mu4e-view-use-old (which must be set before starting mu4e).

Update documentation / mentions.

Load the correct view when starting mu4e, so people can customize
e.g. the keymap.

Add some sanity checking.

mu4e/mu4e-icalendar.el

@@ -58,6 +58,9 @@
 (require 'mu4e-view)
 (require 'mu4e-vars)
 
+(when mu4e-view-use-old
+  (mu4e-error "iCalender support is not available with the old viewer"))
+
 ;;;###autoload
 (defun mu4e-icalendar-setup ()
   "Perform the necessary initialization to use mu4e-icalendar."